CN113406546B - Anti-eddy-current-effect transmembrane water exchange magnetic resonance imaging sequence design method - Google Patents
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Abstract
The invention discloses a design method of an anti-eddy-current-effect transmembrane water exchange magnetic resonance imaging sequence, and provides an innovative design method of a radio-frequency intermittent excitation palindrome symmetric repeated echo sequence to achieve the aim of anti-eddy-current effect. By adopting the innovative sequence design method, three sequence design combinations of a filtering unit and a detecting unit in the anti-vortex transmembrane water exchange magnetic resonance imaging sequence are realized. In the first combination, the filtering unit is designed by using 90-degree and 180-degree radio frequency intermittent excitation palindromic symmetric repeated echo sequences, and the detecting unit is designed by using a conventional pulse gradient spin echo sequence. In the second combination, the filter unit is designed by using a conventional pulse gradient spin echo sequence, and the detection unit is designed by using 90-degree and 180-degree radio frequency intermittent excitation palindromic symmetric repeated echo sequences. In the third combination, the filter unit and the detection unit are designed by adopting 90-degree and 180-degree radio frequency intermittent excitation palindromic symmetric repeated echo sequences.
Description
Technical Field
The invention relates to the technical field of magnetic resonance sequences, in particular to a design method of a transmembrane water exchange magnetic resonance imaging sequence with an anti-eddy effect.
Background
The human body contains a large amount of water molecules and is widely distributed in microenvironments such as intracellular space, extracellular matrix and the like. Water molecules inside and outside the cell can realize cross-cell membrane exchange through phospholipid bilayers or aquaporins on cell membranes, and the exchange rate is in direct proportion to the permeability of the cell membranes to the water molecules. In the course of edema associated with physiological abnormalities in the human body, cell membrane permeability plays a key role. By quantifying the water exchange rate, changes in cell membrane permeability under different physiological conditions can be reflected. Therefore, it is of great value to develop a non-invasive transmembrane water exchange magnetic resonance imaging method which can be applied to a living body.
Existing transmembrane water exchange magnetic resonance imaging sequences are based on conventional diffusion weighted imaging sequences. Because the diffusion coding gradient has high amplitude and long duration, when the diffusion coding gradient is switched, the variable gradient magnetic field can enable the electric conductor around the gradient coil to generate eddy current, and the eddy current further generates a magnetic field for blocking the change of the gradient magnetic field, so that the gradient waveform is deformed. From the perspective of magnetic resonance signal analysis, the eddy current induced gradient waveform deformation in turn shifts the phase of the magnetic resonance signal, which appears as image distortion on the magnetic resonance image. When diffusion coding gradients are applied in different directions, image distortion occurs in different directions, and the image distortion has a directional dependence. In order to quantify the transmembrane water exchange rate of rotation invariance, images acquired by applying diffusion coding gradients in different directions need to be combined, and in this case, the problem of image mismatching is necessarily caused, and the accurate measurement of the transmembrane water exchange rate is influenced. Therefore, in summary, the eddy current effect in the existing design of the trans-membrane water exchange magnetic resonance imaging sequence can generate significant image distortion, and the accuracy and stability of quantifying the water exchange rate are insufficient.
The eddy effect cannot be completely eliminated, and at present, a strategy for reducing the influence of the eddy effect is mainly adopted. In existing transmembrane water exchange sequence designs, two methods can be employed to reduce the vortex effect. One approach is to compensate for eddy current effects on a hardware basis prior to executing the sequence. Currently, most manufacturers of magnetic resonance adopt a self-shielding gradient coil in combination with a gradient waveform pre-emphasis technique. Self-shielded gradient coils compensate for eddy currents by applying currents opposite to the gradient coils, however, self-shielded gradient coils cannot eliminate significant time-varying eddy current fields. The gradient waveform pre-emphasis technology is to pre-process the gradient waveform to pre-distort the waveform to a certain degree, and to make the actually output gradient waveform reach the expectation after eddy current attenuation, but this technology can only compensate the linear term of the eddy current field to a certain extent, and cannot process the non-linear term. Even with the simultaneous use of self-shielded gradient coils and gradient waveform pre-emphasis techniques, significant image distortion still exists on the magnetic resonance image. Another approach is to register the images during the post-image processing stage to correct for image distortions caused by eddy currents. However, the post-processing method depends on a specific algorithm, can only operate on the image level, and cannot directly act on the eddy current effect.
Disclosure of Invention
The invention aims to solve the defects in the prior art, and provides a design method of a transmembrane water exchange magnetic resonance imaging sequence with an anti-eddy effect, which is used for reducing the eddy effect, reducing image distortion and improving the measurement accuracy of the water exchange rate.
The purpose of the invention can be achieved by adopting the following technical scheme:
a design method of an anti-eddy-effect transmembrane water exchange magnetic resonance imaging sequence is disclosed, the anti-eddy-effect transmembrane water exchange magnetic resonance imaging sequence is sequentially composed of a filtering unit, a detecting unit and a mixing unit, and the design method comprises the following steps:
s1, designing 90-degree and 180-degree radio frequency intermittent excitation palindromic symmetric repeated echo sequences;
s2, determining the sequence design combination of the filtering unit and the detecting unit;
s3, under the selected sequence design combination, calculating diffusion coding gradient pulses in the filtering unit and the detecting unit according to the expected diffusion weighting factor;
s4, calculating the breaking gradient pulses in the filtering unit and the detecting unit according to the expected breaking gradient dephasing;
s5, determining the duration of the mixing unit, and calculating a phase-winding gradient pulse according to the expected phase-winding gradient dephasing;
and S6, determining a data reading sequence in the detection unit.
Further, the 90 ° and 180 ° rf intermittent excitation palindromic repeat echo sequence is composed of two 90 ° excitation rf pulses, two 180 ° refocusing rf pulses, and two sets of bipolar diffusion encoding gradients, and the 90 ° and 180 ° rf intermittent excitation palindromic repeat echo sequence has a palindromic repeat structure.
Further, the sequence design combination of the filtering unit and the detecting unit comprises the following three types: in the first combination, the filter unit adopts 90-degree and 180-degree radio frequency intermittent excitation palindromic symmetric repeated echo sequence design, and the detection unit adopts conventional pulse gradient spin echo sequence design; in the second combination, the filter unit is designed by adopting a conventional pulse gradient spin echo sequence, and the detection unit is designed by adopting 90-degree and 180-degree radio frequency intermittent excitation palindromic symmetric repeated echo sequences; in a third combination, the filter unit and the detection unit are designed by adopting 90-degree and 180-degree radio frequency intermittent excitation palindromic symmetric repeated echo sequence.
Further, in the step S3, the diffusion encoding gradient pulse is calculated according to the following relation:
b＝γ 2 G d 2 δ 2 (Δ-δ/3)
where γ is the gyromagnetic ratio, b is the diffusion weighting factor, G d For the diffusion coding gradient magnitude, δ is the duration of the diffusion coding gradient and Δ is the time interval between the start of two diffusion coding gradients.
Further, in the step S4, the gradient pulse is calculated according to the following relation:
in the formula (I), the compound is shown in the specification,
for breaking up the gradient dispersed phase, G c For breaking gradient amplitude, t c For the fracture gradient duration, the fracture gradient is applied in the z-direction, Δ z being the voxel size in the z-direction. Within a horizontally disposed main magnet 10, it is generally defined that the z-direction is the direction toward the viewer, the y-direction is the bottom-up direction, and the x-direction is the left-to-right direction.
Further, in step S5, the spoiler gradient pulse is calculated according to the following relation:
in the formula (I), the compound is shown in the specification,
for perturbing the gradient dephasing, G s Amplitude of the spoiler gradient, t s For spoiling gradient duration, the breaking gradient can be applied in any of the z, y and z directions, Δ r being the voxel size in the selected direction.
Further, in step S6, the data readout in the detection unit adopts an echo planar imaging sequence.
Further, the conventional pulse gradient spin echo sequence consists of one 90 ° excitation rf pulse, one 180 ° refocusing rf pulse, and a set of unipolar diffusion encoding gradients.
Compared with the prior art, the invention has the following advantages and effects:
(1) the sequence design method disclosed by the invention is simple and feasible, and does not need additional hardware equipment and a special processing algorithm.
(2) The sequence design method disclosed by the invention can be completely compatible with the prior art. On the basis of applying a hardware-based eddy current compensation technology, by optimizing the application mode of diffusion coding gradients in a transmembrane water exchange magnetic resonance imaging sequence, including the number, polarity and amplitude of the diffusion coding gradients, the eddy current effect generated by switching of the diffusion coding gradients is restrained from the source, and the eddy current compensation effect is greatly improved. After the sequence design method is used for acquiring data and reconstructing images, the possible residual image distortion can be further reduced by matching with a post-processing algorithm.
Drawings
FIG. 1 is a schematic diagram of the structure and operation of a magnetic resonance imaging system according to the present disclosure;
FIG. 2 is a timing diagram of a conventional transmembrane water exchange magnetic resonance imaging sequence;
FIG. 3 is a schematic diagram of a first sequence design combination of a filter unit and a detection unit according to the present invention;
FIG. 4 is a schematic diagram of a second sequence design combination of a filter unit and a detection unit according to the present invention;
FIG. 5 is a schematic diagram of a third sequence design combination of a filter unit and a detector unit according to the present invention.
Detailed Description
In order to make the objects, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the drawings in the embodiments of the present invention, and it is obvious that the described embodiments are some, but not all embodiments of the present invention. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.
Examples
Fig. 1 shows a component structure of a magnetic resonance imaging system suitable for the sequence design method proposed by the present invention. The key components of the magnetic resonance imaging system comprise a computer 1, a magnetic resonance imaging scanning device 2, a pulse sequence generator 3, a gradient driving module 4, a radio frequency driving module 5, a data acquisition module 6, a data processing module 7, a scanning room interface module 8 and a scanning object positioning module 9. The implementation of the transmembrane water exchange magnetic resonance imaging sequence provided by the invention completely depends on the implementation of each key component of the equipment. Therefore, the following first describes specific embodiments of the key components, and on the basis thereof, describes specific embodiments of the transmembrane water exchange magnetic resonance imaging sequence.
The computer 1 mainly includes a main computer, a radio frequency pulse controller and a gradient pulse controller. The computer 1 communicates and exchanges data in real time with three modules in the magnetic resonance imaging system, mainly a pulse sequence generator 3, a data acquisition module 6 and a data processing module 7, through specific interfaces. The magnetic resonance imaging scanner 2 basically includes a main magnet 10 for generating a main magnetic field, gradient coils 11 for generating gradient magnetic fields, and a radio frequency coil (a birdcage coil 12 or an array coil 13) for generating a radio frequency magnetic field). The main magnet 10 generates a main magnetic field that is constant over time, polarizing the nuclear spins of the scanned subject within the imaging region. The sum of the vectors of the polarized nuclear spins in a unit volume is the magnetization vector, which coincides with the direction of the main magnetic field, usually the z-direction. The gradient coil 11 comprises three separate sub-coils for generating a gradient magnetic field G in the z, y and z directions, respectively x 、G y And G z . The radio frequency coil adopts a birdcage coil 12 or an array coil 13, wherein the birdcage coil 12 can be used as a transmitting radio frequency coil or a receiving radio frequency coil, and the array coil 13 can only be used as a receiving radio frequency coil.
The pulse sequence generator 3 generates digital signals of the gradient pulse waveform and the radio frequency pulse waveform, and transmits the digital signals to the gradient driving module 4 and the radio frequency driving module 5. The gradient driving module 4 generates gradient pulses according to the digital signals, applies the gradient pulses to the gradient coil 11, and generates a gradient magnetic field. The radio frequency driving module 5 generates radio frequency pulses according to the digital signals, applies the radio frequency pulses to the birdcage coil 12 serving as a transmitting radio frequency coil, and generates an oscillating radio frequency magnetic field so that the magnetization vector deviates from the main magnetic field direction. The radio frequency driving module 5 controls a birdcage coil 12 or an array coil 13 as a receiving radio frequency coil to induce a radio frequency magnetic field generated by the nuclear spins and generate a magnetic resonance signal. The data acquisition module 6 acquires magnetic resonance signals in real time according to the sequence time sequence information of the pulse sequence generator 3, and the data processing module 7 carries out reconstruction and post-processing. The pulse sequence generator 3 communicates with the scan room interface module 8, and controls the scan object positioning module 9 to deliver the scan object to a specific position in the magnet.
The sequence design method is adopted by the pulse sequence generator 3, and a transmembrane water exchange magnetic resonance imaging sequence with an anti-eddy effect can be generated. The sequence design method will be described in detail below. The following definitions are first accomplished:
Δ f : the time interval between the application of the diffusion coding gradient in the filter unit. For 90 DEG and 180 DEG radio frequency intermittent excitation palindromic symmetric repeating echo sequences, Delta f Refers to the time interval between the starting points of two sets of bipolar diffusion coding gradients. For the conventionPulse gradient spin echo gradient sequence, Δ f Refers to the time interval between the start of two homopolar diffusion coding gradients.
Δ d : the time interval between the application of diffusion coding gradients in the "detection module". For 90 DEG and 180 DEG radio frequency intermittent excitation palindromic symmetric repeating echo sequences, Delta d Refers to the time interval between the starting points of two sets of bipolar diffusion coding gradients. For conventional pulse gradient spin echo gradients, Δ d Refers to the time interval between the start of two homopolar diffusion coding gradients.
G f And G d : the magnitude of the diffusion coding gradient in the filter unit and the detection unit.
G c : a detection unit and an amplitude of a fragmentation gradient in the detection unit.
G s : the amplitude of the spoiler gradient in the mixing unit.
b f And b d : diffusion weighting factors of the filtering unit and the detecting unit.
Transverse magnetization vector: a detectable component of the magnetization vector is generated in the x-y plane perpendicular to the direction of the main magnetic field (z-direction).
In the sequence design method, a 90-degree and 180-degree radio frequency intermittent excitation palindromic symmetric repeated echo sequence needs to be designed. The 90 DEG and 180 DEG radio frequency intermittent excitation palindromic symmetric repetition echo sequence is composed of two 90 DEG excitation radio frequency pulses, two 180 DEG refocusing radio frequency pulses and two groups of bipolar diffusion coding gradients, and has a palindromic symmetric repetition structure. The amplitude of each diffusion coding gradient in the 90 DEG and 180 DEG radio frequency intermittent excitation palindromic symmetric repeated echo sequence is the same, but the polarity is changed alternately, by changing the polarity of the diffusion coding gradients, eddy currents generated by the switching of different diffusion coding gradients are mutually counteracted, and the final eddy currents are attenuated to a smaller value.
Then, a sequence design combination of the filter unit and the detection unit is determined. Fig. 2 shows a timing diagram of a conventional transmembrane water exchange magnetic resonance imaging sequence, wherein a conventional pulse gradient spin echo sequence design is adopted by a filtering unit and a detecting unit, and the sequence has a remarkable eddy current effect. The invention emphasizes on adopting an innovative design method of a radio frequency intermittent excitation palindrome symmetric repeated echo sequence to realize the anti-vortex-effect transmembrane water exchange magnetic resonance imaging sequence.
The anti-vortex transmembrane water exchange magnetic resonance imaging sequence design method disclosed by the invention realizes the sequence design combination of three filtering units and a detection unit. In the first combination, the filtering unit is designed by using 90-degree and 180-degree radio frequency intermittent excitation palindromic symmetric repeated echo sequences, and the detecting unit is designed by using a conventional pulse gradient spin echo sequence. In the second combination, the filter unit is designed by using a conventional pulse gradient spin echo sequence, and the detection unit is designed by using 90-degree and 180-degree radio frequency intermittent excitation palindromic symmetric repeated echo sequences. In a third combination, the filter unit and the detection unit are designed by adopting 90-degree and 180-degree radio frequency intermittent excitation palindromic symmetric repeated echo sequence.
Then, diffusion encoding gradient pulses in the filtering unit and the detecting unit are calculated according to the expected diffusion weighting factors.
Fig. 3 shows a first sequence design combination of a filter unit and a detection unit. For the filtering unit, a 90-degree and 180-degree radio frequency intermittent excitation palindromic symmetric repeated echo sequence design is adopted. Two sets of bipolar gradients ( gradients 14, 15, 16 and 17) are used as diffusion encoding gradients, the first set of bipolar gradients comprising a positive gradient 14 and a negative gradient 15, the second set of bipolar gradients comprising a positive gradient 16 and a negative gradient 17, each set of positive and negative gradients being separated by a 180 ° refocusing radio frequency pulse in the x or y direction, respectively. The diffusion encoding gradients may be applied along any direction, but the direction of all diffusion encoding gradients in a single scan is uniform. The diffusion encoding gradients may be applied in the x, y, or z directions individually, or in any two or all three of the z, y, and z directions simultaneously. The magnitude of all diffusion encoding gradients is G f Respectively of duration delta 1 、δ 2 、δ 3 、δ 4 And satisfies the relation δ 1 +δ 2 ＝δ 3 +δ 4 The time interval between the start points of the positive gradients 14 and 16 is delta f . To produce the desired diffusionWeighting factor b f The following relationship is satisfied:
b f ＝γ 2 G f 2 (δ 1 +δ 2 ) 2 [Δ f -(δ 1 +δ 2 )/3]
for the detection unit, a conventional pulse gradient spin echo sequence design is employed. The diffusion encoding gradient consists of a set of unipolar gradients (positive gradients 18 and 19) separated by a 180 refocusing radio frequency pulse in either the x or y direction. The diffusion coding gradient direction is the same as that of the filtering unit. The positive gradients 18 and 19 are equal in magnitude and duration, G respectively d And delta 5 The time interval between the start points of the positive gradients 18 and 19 is Δ d . To generate a desired diffusion weighting factor b d The following relationships are satisfied:
b d ＝γ 2 G d 2 δ 5 2 [Δ d -δ 5 /3]
fig. 4 shows a second sequence design combination of a filter unit and a detection unit. For the filtering unit, a conventional pulse gradient spin echo sequence design is employed. A set of but polar gradients (positive gradients 20 and 21) make up the diffusion encoding gradients, the positive gradients 20 and 21 being separated by a 180 refocusing radio frequency pulse in either the x or y direction. The direction of the diffusion-encoded gradient is combined with the first. The positive gradients 20 and 21 are equal in magnitude and duration, G respectively f And delta 1 The time interval between the starting points of the positive gradients 20 and 21 is Δ f . To generate a desired diffusion weighting factor b f The following relationships are satisfied:
b f ＝γ 2 G f 2 δ 1 2 [Δ f -δ 1 /3]
for the detection unit, a 90-degree and 180-degree radio frequency intermittent excitation palindromic symmetric repeated echo sequence design is adopted. The positive gradient 22 and the negative gradient 23 form a first set of bipolar gradients, the positive gradient 24 and the negative gradient 25 form a second set of bipolar gradients, the two sets of bipolar gradients form a diffusion encoding gradient, and the two sets of positive and negative gradients are separated by two 180 DEG refocusing radio frequency pulses in the x or y direction respectively. Diffusion ofThe direction of the encoding gradient is combined with the first. The magnitude of all diffusion coding gradients is G d Respectively having a duration of delta 2 、δ 3 、δ 4 And delta 5 And satisfies the relation δ 2 +δ 3 ＝δ 4 +δ 5 The time interval between the start points of the positive gradients 22 and 24 is Δ d . To generate a desired diffusion weighting factor b d The following relationships are satisfied:
b d ＝γ 2 G d 2 (δ 2 +δ 3 ) 2 [Δ d -(δ 2 +δ 3 )/3]
fig. 5 shows a third sequence design combination of a filter unit and a detection unit. For the filtering unit, a 90-degree and 180-degree radio frequency intermittent excitation palindromic symmetric repeated echo sequence design is adopted. The positive gradient 26 and the negative gradient 27 form a first set of bipolar gradients, the positive gradient 28 and the negative gradient 29 form a second set of bipolar gradients, the two sets of bipolar gradients form a diffusion encoding gradient, and the two sets of positive and negative gradients are separated by two 180 DEG refocusing radio frequency pulses in the x or y direction, respectively. The diffusion coding gradient direction is combined with the first one. The gradients 26, 27, 28 and 29 are all of magnitude G f Respectively of duration delta 1 、δ 2 、δ 3 And delta 4 And satisfies the relation δ 1 +δ 2 ＝δ 3 +δ 4 The time interval between the start points of the positive gradients 26 and 28 is Δ f . To generate a desired diffusion weighting factor b f The following relationships are satisfied:
b f ＝γ 2 G f 2 (δ 1 +δ 2 ) 2 [Δ d -(δ 1 +δ 2 )/3]
for the detection unit, a 90-degree and 180-degree radio frequency intermittent excitation palindromic symmetric repeated echo sequence design is also adopted. The positive gradient 30 and the negative gradient 31 form a first set of bipolar gradients, the positive gradient 32 and the negative gradient 33 form a second set of bipolar gradients, the two sets of bipolar gradients form a diffusion encoding gradient, and the two sets of positive and negative gradients are separated by two 180 DEG refocusing radio frequency pulses in the x or y direction, respectively. Diffusion coding gradient methodIn the same first combination. The gradients 30, 31, 32 and 33 are all of magnitude G d Respectively having a duration of delta 5 、δ 6 、δ 7 And delta 8 And satisfies the relation δ 5 +δ 6 ＝δ 7 +δ 8 The time interval between the start points of the positive gradients 30 and 32 is delta d . To generate a desired diffusion weighting factor b d The following relationships are satisfied:
b d ＝γ 2 G d 2 (δ 5 +δ 6 ) 2 [Δ d -(δ 5 +δ 6 )/3]
then, according to the desired fragmentation gradient dephasing, the fragmentation gradient pulses in the filter unit and the detection unit are calculated. The effect of the fragmentation gradient is to eliminate undesirable signal paths. The fragmentation gradient consists of two independent gradients, the first one located after the last diffusion-encoded gradient in the filtration unit and the second one located before the first diffusion-encoded gradient in the detection unit. Both crushing gradients are positive and have amplitude of G c Duration of t c And are all applied in the z-direction. To produce the desired dephasing
The following relationships are satisfied:
where Δ z is the voxel size in the z direction. According to experience, minimum
Typically satisfying more than 4 pi.
In the first and third sequence design combinations, the filtering unit uses 90 ° and 180 ° rf intermittent excitation palindromic repeat echo sequence design, and the fragmentation gradient is already followed by a 90 ° rf pulse. In particular, in the second sequence design combination, it is necessary to follow the disruption gradient of the filter unitAn additional 90 rf pulse in the x-direction is applied. The effect of the 90 ° radio frequency pulse after the fragmentation gradient is to store the transverse magnetisation vector in the x-y plane in the z direction. The time from the center of the 90 DEG radio frequency pulse to the center of the 90 DEG excitation radio frequency pulse in the detection unit is the duration t of the mixing unit m . At t m During this period, the water molecules exchange across the membrane, and the magnetic resonance signal attenuated in the filter unit is gradually restored, the degree of restoration depending on the duration t of the mixing unit m And transmembrane water exchange rate. t is t m The longer the transmembrane water exchange rate, the faster the magnetic resonance signal recovers. In particular, a 90 ° excitation radio frequency pulse in the detection unit flips the magnetization vector stored in the z-direction to the x-y plane again.
A wrap-phase gradient pulse is then calculated based on the desired wrap-phase gradient dephasing. In the hybrid cell, the effect of the phase-wise gradient is to completely dephase the residual transverse magnetization vector in the x-y plane. The spoiler gradient may be applied in the x, y or z directions individually or in all three directions simultaneously. Amplitude of the phase-wound gradient is G s Duration of t s And Δ r is the voxel size in the direction of application of the spoiler gradient. To produce the desired gradient dephasing
The following relationships are satisfied:
usually by arranging loose phases
Greater than 2 pi.
The detection unit acquires data by adopting an echo planar imaging sequence. In a single acquisition, the echo planar imaging sequence is capable of acquiring all the data used to reconstruct the magnetic resonance image, which is one of the most time-efficient data readout sequences at present.
Finally, the pulse sequence generator 3 transmits all gradient pulse waveform and radio frequency pulse waveform data in the filtering unit, the detecting unit and the mixing unit to the gradient driving module 4 and the radio frequency driving module 5. The gradient driving module 4 drives the gradient coil 11 to generate a gradient magnetic field according to a designed timing. The radio frequency drive module 5 drives a birdcage coil 12, which is a transmit radio frequency coil, to generate an oscillating radio frequency magnetic field. The radio frequency driving module 5 drives a birdcage coil 12 or an array coil 13 as a receiving radio frequency coil to induce a radio frequency magnetic field generated by nuclear spins. The sequence formed by the filtering unit, the detecting unit and the mixing unit is the transmembrane water exchange magnetic resonance imaging sequence with the anti-eddy effect generated by the pulse sequence generator 3 by adopting the sequence design method.
The above embodiments are preferred embodiments of the present invention, but the present invention is not limited to the above embodiments, and any other changes, modifications, substitutions, combinations, and simplifications which do not depart from the spirit and principle of the present invention should be construed as equivalents thereof, and all such modifications are intended to be included in the scope of the present invention.
Claims (6)
1. A design method of an anti-vortex-effect transmembrane water exchange magnetic resonance imaging sequence is characterized in that the anti-vortex transmembrane water exchange magnetic resonance imaging sequence adopts a radio frequency intermittent excitation palindromic symmetric repeated design method, the anti-vortex transmembrane water exchange magnetic resonance imaging sequence is sequentially composed of a filtering unit, a detecting unit and a mixing unit, and the design method of the anti-vortex transmembrane water exchange magnetic resonance imaging sequence comprises the following steps:
s1, designing 90-degree and 180-degree radio frequency intermittent excitation palindromic symmetric repeated echo sequences; the 90-degree and 180-degree radio frequency intermittent excitation palindromic symmetric repeated echo sequence is composed of two 90-degree excitation radio frequency pulses, two 180-degree refocusing radio frequency pulses and two groups of bipolar diffusion coding gradients, and the 90-degree and 180-degree radio frequency intermittent excitation palindromic symmetric repeated echo sequence has a palindromic symmetric repeated structure;
s2, determining the sequence design combination of the filter unit and the detection unit; the sequence design combination of the filtering unit and the detecting unit comprises the following three types: in the first combination, the filter unit adopts 90-degree and 180-degree radio frequency intermittent excitation palindromic symmetric repeated echo sequence design, and the detection unit adopts conventional pulse gradient echo sequence design; in the second combination, the filter unit is designed by adopting a conventional pulse gradient echo sequence, and the detection unit is designed by adopting 90-degree and 180-degree radio frequency intermittent excitation palindromic symmetric repeated echo sequences; in the third combination, the filtering unit and the detecting unit are designed by adopting 90-degree and 180-degree radio frequency intermittent excitation palindromic symmetric repeated echo sequences;
s3, under the selected sequence design combination, calculating diffusion coding gradient pulses in the filtering unit and the detecting unit according to the expected diffusion weighting factor;
s4, calculating the crushing gradient pulse in the filtering unit and the detecting unit according to the expected crushing gradient dephasing;
s5, determining the duration of the mixing unit, and calculating a phase-winding gradient pulse according to the expected phase-winding gradient dephasing;
and S6, determining a data reading sequence in the detection unit.
2. The method for designing an anti-eddy current effect transmembrane water exchange mri sequence according to claim 1, wherein in step S3, the diffusion encoding gradient pulses are calculated according to the following relation:
b＝γ 2 G d 2 δ 2 (Δ-δ/3)
where γ is the gyromagnetic ratio, b is the diffusion weighting factor, G d For the diffusion coding gradient magnitude, δ is the duration of the diffusion coding gradient and Δ is the time interval between the start of two diffusion coding gradients.
3. The method for designing an anti-eddy current effect transmembrane water exchange mri sequence according to claim 1, wherein in the step S4, the fragmentation gradient pulse is calculated according to the following relation:
wherein gamma is a gyromagnetic ratio,
for breaking up the gradient dispersed phase, G c For breaking gradient amplitude, t c For the duration of the fragmentation gradient, the fragmentation gradient is applied in the z-direction, Δ z being the voxel size in the z-direction, which is defined as the direction pointing towards the observer in a horizontally placed main magnet.
4. The method for designing an anti-eddy current effect transmembrane water exchange mri sequence according to claim 1, wherein in the step S5, the spoiler gradient pulse is calculated according to the following relation:
wherein gamma is a gyromagnetic ratio,
for perturbing the gradient dephasing, G s Amplitude of the spoiler gradient, t s For spoiler gradient duration, the breaker gradient can be applied in any of x, y and z directions, Δ r being the voxel size in the selected direction, defining the z direction as the direction pointing towards the viewer, the y direction as the bottom-up direction, and the x direction as the left-to-right direction, within a horizontally placed main magnet.
5. The method for designing an anti-eddy current effect transmembrane water exchange mri sequence according to claim 1, wherein in step S6, the data readout in the detection unit employs an echo planar imaging sequence.
6. A method for designing an anti-eddy current effect transmembrane water exchange mri sequence according to claim 1, wherein the conventional pulse gradient spin echo sequence is composed of a 90 ° excitation rf pulse, a 180 ° refocusing rf pulse, and a set of unipolar diffusion encoding gradients.
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| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN104459587A (en) * | 2013-09-17 | 2015-03-25 | 华润万东医疗装备股份有限公司 | Flow compensation method for nuclear magnetic resonance imaging system |
| CN110739059A (en) * | 2018-07-18 | 2020-01-31 | 西门子医疗有限公司 | Generating setting parameters for MRT sequences |
| CN111751770A (en) * | 2020-06-12 | 2020-10-09 | 浙江大学 | Method for quantitatively measuring exchange speed of water molecules inside and outside blood vessels by means of non-magnetic resonance contrast agent |
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| ES2822201T3 (en) * | 2014-06-12 | 2021-04-29 | Commissariat Energie Atomique | MRI method to quantify the amount of iron in tissues using diffusion magnetic resonance imaging |
| US11826134B2 (en) * | 2019-06-13 | 2023-11-28 | University Of Southern California | Method for measuring water exchange across the blood-brain barrier using MRI |
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| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN104459587A (en) * | 2013-09-17 | 2015-03-25 | 华润万东医疗装备股份有限公司 | Flow compensation method for nuclear magnetic resonance imaging system |
| CN110739059A (en) * | 2018-07-18 | 2020-01-31 | 西门子医疗有限公司 | Generating setting parameters for MRT sequences |
| CN111751770A (en) * | 2020-06-12 | 2020-10-09 | 浙江大学 | Method for quantitatively measuring exchange speed of water molecules inside and outside blood vessels by means of non-magnetic resonance contrast agent |
Non-Patent Citations (4)
| Title |
|---|
| Noninvasive mapping of water diffusional exchange in the human brain using filter-exchange imaging;Nilsson M, et al.;《Magnetic Resonance in Medicine》;20120726;第69卷(第6期);第1573-1581页 * |
| Reduction of eddy-current-induced distortion in diffusion MRI using a twice-refocused spin echo;Reese TG, et al.;《Magnetic Resonance in Medicine》;20030101;第49卷(第1期);第177-182页 * |
| 基于水扩散的定量磁共振成像研究;李晓东;《中国优秀硕士学位论文全文数据库 医药卫生科技辑》;20210515(第05期);第E060-56页 * |
| 快速成像序列设计与伪影消除方法研究;黄佳佳;《中国优秀硕士学位论文全文数据库 医药卫生科技辑》;20160315(第03期);第E060-78页 * |
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